The drive to reduce the overall thickness and increase the routing density in electronic packaging has necessitated that the line widths and spacing between copper lines be reduced. In order to obtain the reduced line widths and spacing, the thicknesses of the copper lines must also be reduced. A drawback to reducing the thickness of copper lines is that the path resistance (Rpath) is increased. Maintaining a low Rpath is particularly important in the design of power delivery networks within the package.
Ideally, the power planes in the power delivery network are designed with a minimum resistance and inductance. These parameters may be minimized by increasing the thickness of the power plane. However, since the copper lines and the power plane are formed with the same patterning and metal deposition processes, thicker metal in the power plane requires the thickness of the copper lines used for signal routing to be increased as well. Accordingly, when a thicker power plane is used, the signal routing lines in the layer require larger minimum line widths and spacings. The increased line width and spacing affects signal routing density and therefore, requires an increase in the number of layers of the package. Increasing the number of layers increases the overall thickness of the package and increases the cost of the package.
Thus, improvements are needed in the area of electronic packaging fabrication in order to form different metal thickness within a single layer in order to provide thick metal for the power plane routing and thin metal for the signal routing.